Process for improving the color of wood pulp



Patented Feb. 16, 1937 PATENT OFFICE PROCESS FOR IMPROVING THE COLOR F WOOD PULP Hugh Bryson Glass, Birmingham, Ala.

No Drawing.

Application May 10, 1935,

Serial N0. 20,755

3 Claims.

My invention relates to a process for improving the color of vegetable fiber pulp, and is particularly applicable to improving the color of pulp produced from pine wood by the soda. or sulphate process.

The principal objects of this invention are to provide a process of the character designated which shall be economical as to time and materials employed; which shall not materially weaken the fiber; and by means of which paper pf any desired shade from the well known kraft brown to white may be readily produced.

It is well known that cellulose is the structural material of plants comprising, as it does,

5 the walls of the cells in which the chemical processes of plant life take place; that when pure it is white in color and quite resistant to the action of most chemicals, and that in the raw materials for its production, as obtained from plants, cellulose is associated with other materials," such as gums, waxes, resins, lignins, polysaccharides, etc.

In the soda or sulphate process of treating cellulose, these substances are partially separated 25 and materially altered in their chemical constitution. The product resulting from the treatment is generally of a light brown color containing a considerable amount of coloring matter.

Chlorine, in the gas phase, in aqueous solution, or combined with bases as hypochlorites, has long been used for the bleaching of mechanical or chemical wood pulp. A number of methods for using this substance in one or more of the above forms have been proposed and savings in the 35 amount of chlorine required, in the quantity of cellulose utilized, and in the preserving of the original quality or strength of the cellulose have been accomplished. (See Kress and Davis, Paper Trade J. 94, No. 11, 34-40 (1932), Hansen, Ibid. 40 95, No. 3, 30-2 (1932), JohnU. S. 1,780,943-

11/11/31, Rys-U. S. 1,787,395--12/30/30, Curran and Bray, Paper Trade J. 92, No. 2, 45-52 (1931), Hagglund-U. S. 1,792,009--2/10/31, Broderick-U. S. 1,792,805-2/17/31, Bradley and 5 McKeefeU. S. 1,768,820-1-2-3 7/l/30, Heuser, Paper Pulp Mag. Can. 30, 101-4 v(1930) etc.) I have found that by treating the brown cellulose fiber from the soda or sulphate process in the manner hereinafter described and with 50 careful control of the hydrogen ion concentrations of the baths employed, within the limits specified, that the major portion of the coloring matter within the fibers is removed, as distinguished from well known bleaching operations. 55 The relatively small percentage of coloring matter which remains in the fiber may be readily and inexpensively bleached to give the desired color.

Briefly, my invention comprises the treatment of kraft pulp, first with a solution containing 5 hypochlorous acid in which free hydrochloric acid has been neutralized, followed by a treatment with an alkaline solution containing no chlorine or other so-called bleaching agent, and a final step in which the pulp is again subject- 10 ed to a solution similar to that first mentioned. In carrying out my improved process, I have found that the first treatment mentioned apparently brings about a reaction between the coloring matter in the pulp and the acid, whereby the coloring matter is rendered soluble in alkalies which are free from chlorine or other bleaching agents, whereby the coloring matter may be largely removed by washing in the alkaline solution. The alkaline solution, when drained off from the pulp in the second step, is found to contain a large amount of coloring matter, and the pulp remaining is found to contain a relatively small percentage of its original coloring matter. In the third step of my process, wherein the pulp is again treated with a solution containing hyprochlorous acid in which free hydrochloric acid has been neutralized, the small percentage of remaining coloring matter is apparently bleached. The degree of coloring matter removed, and consequently the ultimate color of the product obtained, can be governed within any practical limits desired by the operator of the process, whereby paper manufactured from the pulp may be produced in any gradations of shade from the kraft brown to a, high white.

In carrying out my process, I first prepare the acid solution by dissolving chlorine in water in a suitable vessel in which a quantity of a water insoluble alkaline earth carbonate is placed, such as crushed limestone or dolomite. As is well known, the chlorine goes into solution in the water forming hydrochloric and hypochlorous acid, according to the following equation:

It is also Well known that the hydrochloric acid will react with the carbonate to form calcium or magnesium chloride in solution so that the resulting solution contains free hypochlorous acid, carbonic acid, and with substantially no free hydrochloric acid. In making up this solution, a. convenient strength is around 5 grams of chlorine per liter, but obviously lower or higher concentrations may be employed. Since carbonic acid is a stronger acid than hypochlorous, the

acidity of the solution is largely governed by the presence of the carbonic acid. In carrying out my process I have not found it necessary or desirable to remove the carbonic acid from the solution. The method of making up this solution is such that it automatically adjusts the pH value of the solution to around 5.0. The solution just described may also be made up by employing any alkaline material which will react with the hydrochloric acid, for example, caustic soda, potash, ammonia, etc., until the desired pH value is reached. Obviously, however, the more economical and more readilr, operated method is that described employing limestone or dolomite.

Where the pulp to be treated is what is known as "hard cooked, 100 parts of the ordinary kraft stock are subjected to the water solution of HOCl,

prepared as above set forth, so that themixture contains approximately 8 parts of chlorine to the 100 parts of pulp, one-half of which is in the form of H001, and sufiicient water to maintain the desired consistency of the suspension. When so mixed, such a mixture will he found to have a pH between 5 and 6. As the reaction proceeds,

the pH decreases and it should not be allowed to go below 4.5.

It is very important that the hydrochloric acid in the aqueous chlorine solution be neutralized before mixing with the pulp. While the nature of the reactions which occur are as yet not understood by me, I have had demonstrated by numerous trials that, with free hydrochloric acid present and with a consequent initial pH lower than 4.5, the coloring matter in the fiber, while bleached to some extent, can not be removed from the pulp as hereinafter described.

The consistency of the pulp, or ratio of dry cellulose to water in the mixture, is not an important factor and is governed solely by the type of equipment and problems of handling. The temperature of the mixture is preferably maintained at around 35 C.

If the temperature of the mixture described be maintained at around 35 C., the time for the reaction to proceed to the desired point will be found to be around twenty minutes. If the temperature is lower than 35 C., a longer time is necessary, and if higher than 35 C. a shorter time is necessary. During the carrying out of this first stage of my improved process, the only agitation necessary is that sufflcient to prevent segregation of the pulp.

As the reaction proceeds, it will be observed that the pulp becomes lighter in color in a very short time. If the pulp and the solution are allowed to stand in contact after the reaction is substantially completed, the pH value of the solution will be observed to decrease rapidly and a reverse color change occurs, the color becoming darker. The pulp should be removed from the solution before this change takes place and should be washed to remove the adhering solution, only a comparatively small quantity of water being required for this purpose. If the pH value ofthe solution is allowed to rise above 6.0, the efficiency of the process will be impaired, while if allowed to decrease below 4.5, the color will be very difficult to remove.

If the pulp be what is known in the trade as soft cooked", that is, a pulp which contains less coloring matter, a lower percentage of chlorine, as 11001, is employed. Samples of soft cooked kraft pulp have been successfully treated with as stock, depending upon the character of the orig-- inal stock and the degree of color removal desired.

The pulp, after being treated in the first stage as heretofore described, and washed, is placed in an alkaline solution having a pH value of about 11.0. Inasmuch as in the second stage of my improved process the coloring matter is being dissolved and leached out, I employ no chlorine compounds or other so-called bleaching agents. The mixture is agitated, as by stirring, and maintained at a temperature between 40 and 60 C. for from to '75 minutes, depending on the kind of stock used and the quantity of chlorine utilized by the stock in the first step.

The alkaline solution is preferably made up by dissolving sodium carbonate in water, for'the reason that extra amounts of the carbonate do not materially affect the pH value. In practice I have had good results by dissolving soda ash in water in the proportions of about one part of the soda ash to 100 parts of water, to give the desired pH value. It will be obvious that any other alkali, such as caustic soda, caustic potash, ammonia or lime may be used, although the control of the pH value will be more difficult.

During the reactions which occur in this second stage of my process, the alkaline solution becomes very dark, almost black, in color, from the coloring matter leached out of or dissolved from the pulp in this operation. Also, the pulp removed from the solution, before washing, will appear darker in color than the original material. When thoroughly washed with water, however, the pulp from this second stage assumes a grayish color, showing that the coloring matter in the pulp has been largely removed.

I have observed that the solvent action by the alkali solution in this second stage of my process is very sensitive to its hydrogen ion concentration. The action of a solution having a pH value lower than 10.5 becomes ineffective as a solvent for the coloring matter while a solution having a pH value greater than 11.5 damages or degrades the quality of the fiber so as to weaken it. In order to obtain satisfactory results it is necessary to maintain a pH value of the solution between these limits or at about 11.0.

At the end of this second stage of my process, the alkaline liquor is drained oil for reuse or recovery of the alkaliand the pulp is thoroughly washedsin clear water to remove the coloring matter as just described.

In the third stage of my improved process, the washed pulp from the second stage is again treated, essentially as in the first stage, that is, with a solution of H001 containing substantially no free hydrochloric acid, and having a pH value between 4.5 and 6.0. During this stage, there is noticed a very rapid change of color of the fibers from a gray or gray brown to a yellow or yellow brown. After the pulp has remained in the solution for from 30 to 90 minutes, the color will have changed to a white or substantially white. After this the pulp is thoroughly washed and is ready for processing. As in the first stage of my process, the third stage should be carried out at a temperature around 35 C. If the solution is maintained at a lower temperature, a longer time is required, while if at a higher temperature, a somewhat shorter time will produce the desired results. In the third stage, however, I have found that there is no danger in over-subjecting the pulp to the acid solution, as no reverse color change has ever been observed.

As stated above, the reaction in the third stage will be found to go to its completion in from thirty to ninety minutes. It will sometimes be found, however, with hard cooked pulp, that the desired whiteness has not yet been attained in the three steps described. In such event, the color removal may be continued by again'subjecting the pulp from the third stage to an alkaline wash as described for the second stage, followed by a final treatment in a solution of hypochlorous acid, as described for the first and third stages. It will be observed that it is a characteristic of my improved process that it consists in an uneven number of steps with three as a minimum, the odd numbered steps being the treatment of the pulp with an acid solution, and the even numbered steps being the treatment of the pulp with an alkaline solution containing no chlorine or other so-called bleaching agent.

From the foregoing it will be apparent that I have devised an improved process for improving the color of wood pulp which distinguishes over prior processes in that the results accomplished are effected largely by the removal of the major portion of the coloring matter from the pulp as highly colored material and less by bleaching or destruction of the coloring material, the only bleaching being that necessary to lighten the color of the undissolved coloring matter. It will furthermore be apparent that my improved process is carried out by the use of relatively weak reagents which have slight, if any, effect upon the strength of the fiber.

While I have shown my invention in but one form, it will be obvious to those skilled-in the art that it is not so limited, but is susceptible of various changes and modifications, without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are imposed by the prior art or as are specifically set forth in the appended claims.

What I claim is:

l. A method of bleaching wood pulp which comprises first subjecting the pulp. to an aqueous HOCl solution in which the free HCl has been neutralized, until the pulp becomes lighter in color, then washing with an alkaline solution containing no chlorine having a pH between 10.5 and 11.5 to remove the alkali soluble coloring matter from the pulp, and finally again subjecting the pulp to the action of an aqueous HOCl solution in which the free HCl has been neutralized, both of said HOCl solutions having a pH not exceeding 6.0 and not being allowed to drop below 4.5.

2. A process as set forth in claim 1, in which the second and last steps are repeated until the pulp is bleached to the desired degree, the final treatment of the pulp being with an aqueous solution of HOCl.

the temperature of the mixture during the acid treatment is maintained around C.. and during the alkaline treatment at from 40 to C.

HUGH BRYSON GLASS.

30 i 3. A process as set forth in claim 1, in which 

